ui: theme tokens follow the configured theme, not the viewer's OS scheme

st.context.theme.type reports the OS preference ('dark' on a dark desktop)
even though Streamlit renders the single custom light theme, so dark tokens
were injected onto a light page (invisible title, black tiles). Use
st.get_option('theme.base') — the config is the one source of truth.

agent/ui_theme.py

@@ -53,10 +53,16 @@ MONO = 'ui-monospace, SFMono-Regular, Menlo, Consolas, "Liberation Mono", monosp
 
 
 def is_dark() -> bool:
-    """Streamlit's active theme type ('light' unless the viewer picked dark)."""
+    """True only when the *configured* Streamlit theme is dark.
+
+    Deliberately NOT st.context.theme.type: with a single custom light theme
+    that call reports the viewer's OS preference ("dark" on a dark desktop)
+    while Streamlit still renders the light theme — injecting dark tokens on
+    a light page. The config is the one source of truth for the whole app.
+    """
     try:
-        return (st.context.theme.type or "light") == "dark"
-    except Exception:          # AppTest / very old Streamlit
+        return (st.get_option("theme.base") or "light") == "dark"
+    except Exception:
         return False